Most tissues of the human body are soft tissues; these tissues are inherently flexible and readily deformable. Further, many of these soft tissues interface with other tissues along boundaries where considerable movement may take place. During surgery, as adjacent structures such as bone are moved and pressure applied with instruments such as retractors, these tissues will deform and shift. Since these tissues may deform readily both between imaging and surgery, and during surgery, it is common for surgeons to find tumors, foreign objects, organs, lesions, and other targets are no longer in the exact positions they occupied in preoperative images.
For a surgeon to properly treat or biopsy surgical targets, the surgeon must locate the correct targets during surgery. Further, for surgeons to avoid unintended damage to other structures, it may also be necessary to locate those other structures precisely during the surgery. While surgeons often locate targets visually, this is not practical when surgical targets are similar in appearance to nearby tissues, are located within organs, are in tight spots close to other organs, when damage to overlying or adjacent structures is to be minimized, or when exposure is otherwise difficult. It is desirable to locate these targets using medical imaging equipment during surgery.
MRI and CT imaging are often used with or without contrast enhancement to provide high resolution preoperative images of surgical targets. The equipment required to make these images is bulky, expensive, and not always easily incorporated into an operating-room environment. Further, the intense magnetic fields required for MRI may be incompatible with other operating room instruments and equipment, and radiation emitted by CT machines may require surgeon and staff leave the room during intraoperative imaging.
Ultrasound imaging equipment is generally less bulky and often far less expensive than MRI and CT equipment. Ultrasound equipment also does not emit electromagnetic radiation or magnetic fields, and is more compatible with the operating room environment. Unfortunately, it may not be possible for some targets to be properly identified in ultrasound images.
In Hartov, et al., Error Analysis for a Free-Hand Three Dimensional Ultrasound System for Neuronavgation, Neurosurgical Focus 6 (3), 5 Aug. 1999, it was suggested that targets found in preoperative MRI or CT images be located during neurosurgery by updating an image with ultrasound and a model of deformation of the brain. In that article, apparatus was proposed that used sensors produced by Ascension Technology Corporation, Milton, Vt., to track a handheld ultrasound transducer in three dimensions. Since not only the position, but the angle of the sensor, could be determined, an object in an image obtained with the transducer could be located to within a few millimeters in three dimensional space.
Other types of tracking devices have also been proposed for determining a location in three dimensions of the ultrasound transducer. For example, the transducer may be mounted to a linkage attached between the transducer and an object, the linkage attached with devices for monitoring motions of the transducer. It is also possible to use an optical tracking system for determining positions and angles of the transducer.